Detector for the detection of x-radiation

ABSTRACT

The invention relates to a detector and a method for the production of consecutive X-ray images. This involves the generation in the detector by X-radiation during an exposure interval (T exp ) of a current flow (I(t)) which is integrated (Q dk *, Q sig *, Q ae *) in a memory capacity and read out in a subsequent readout phase (T rd ). In order to minimize the influence of slowly decaying residual currents (I ae ), the current (I(t)) is integrated only during the exposure interval (T exp ). By this means, disturbing artifacts due to residual signals from earlier photographs are minimized.

The invention relates to a detector for the detection of electromagneticradiation in an exposure interval, in particular of X-radiation. It alsorelates to an X-ray apparatus with such a detector, and to a method forthe detection of electromagnetic radiation, in particular of X-radiation

Used for the detection of X-radiation, are detectors with a conversionelement which converts absorbed X-ray quanta into an electrical signal.Typically the electrical signal involves released electrical chargecarriers (e.g. electron-hole pairs) which are integrated in a memorycapacity by an assigned sensor (pixel). On completion of exposure of theconversion element, the charges contained in the memory capacities areread out by readout electronics and used for the point-by-pointcomposition of an X-ray image. Here the integration of the electricalsignal from the conversion element takes place during the exposureinterval and subsequent readout phase.

With the detectors described above, due to delay in readout ofelectrical charge carriers from the conversion elements, a residualsignal occurs during subsequent photographs, so that the latter containdisturbing artifacts (e.g. ghost images). In order to reduce suchdisturbances, U.S. Pat. No. 6,222,901 B1 proposes a detector with acorrection unit which determines the dark current directly before freshexposure of an X-ray detector and subtracts its value from thesubsequent signals measured under an exposure. Here the subtracted darkcurrent covers the residual signals of earlier exposure periods. Theproblem however is that the residual signals referred to are notconstant during the subsequent photograph, so that the subtraction of aconstant value permits only inaccurate compensation for the residualsignals.

Against this background, the problem of the present invention was toprovide means of detecting electromagnetic radiation, such as inparticular X-radiation, which would reduce or avoid disturbing artifactsdue to residual signals.

This problem is solved by a detector with the features of claim 1, by anX-ray apparatus with the features of claim 7, and by a method with thefeatures of claim 8.

Advantageous Designs are Contained in the Dependent Claims.

The detector according to the invention is used to detectelectromagnetic radiation in an exposure interval, while the radiationmay involve in particular X-radiation.

The Detector Comprises the Following Components:

-   a) One or more sensors with a conversion element for the conversion    of the electromagnetic radiation to be detected into an electrical    signal, and with an integration unit for the integration of said    electrical signal over an integration period. Typically the detector    has a multiplicity of sensors, arranged matrix-like in an array. The    conversion element of the sensor may be set up in particular for the    direct or indirect conversion of X-radiation into electrical charge    carriers (e.g. electron-hole pairs). In the case of indirect    conversion, the X-ray quanta firstly induce in a scintillator the    emission of visible light, which is then converted in a    photodetector into an electrical signal.-   b) A readout circuit for the readout and processing of said    integrated electrical signal from the sensor during a readout    interval which follows the exposure interval.-   c) A control means coupled to the sensor and set up to determine the    specified integration period so that it substantially overlaps the    exposure interval. Such “substantial” overlapping exists when the    exposure interval and the integration period overlap by at least 80%    (based on the longer of the intervals). Preferably the integration    period has a 100% overlap with the exposure interval, i.e. both    intervals have the same starting and finishing point and are    therefore coincident. If there is no such coincidence, then the    integration period typically ends slightly later than the exposure    interval.

Through the matching of exposure interval and integration period, thedetector described achieves a noticeable improvement in the informationcontent of the images made, since the element of residual signals fromprevious photographs in the integrated electrical signal is reduced. Fora detailed explanation of this effect, reference is made to thedescription of the Figures.

The readout circuit of the detector is preferably set up so as tocorrect a dark value of the electrical signal. The dark value of theelectrical signal is to be understood as meaning the magnitude of theelectrical signal observed without any detectable irradiation ofelectromagnetic radiation on the conversion element of the sensor. Anelectrical signal at the level of the dark value thus occurs even duringthe exposure interval irrespective of the detectable electromagneticradiation. So that this element of the electrical signal is noterroneously interpreted as due to radiation, it is deducted from thecurrent electrical signal in the readout circuit. The dark value of theelectrical signal used for the memory capacity may be a static valuewhich occurs after an adequately long dimming of the sensors and thedecay of all residual signals. The dark value may also however bedetermined in accordance with U.S. Pat. No. 6,222,901 B1, in each casecurrently, immediately before a fresh exposure interval, so that in thiscase it also contains elements of residual signals from previousphotographs.

According to a preferred embodiment of the detector, the integrationunit of the sensor has a primary memory capacity, connected electricallyin parallel to the conversion element, while the conversion element isset up for the conversion of electromagnetic radiation into mobileelectrical charge carriers. In this case the primary memory capacity maybe charged up, before an exposure interval, with a preset electricalvoltage which is discharged during the exposure interval with the aid ofthe charge carriers generated in the conversion element The dischargedquantity of charge here corresponds to the integral of the mobilecharges generated in the conversion element.

According to a further feature of the embodiment described above theintegration unit contains a further “secondary” memory capacity, whichis connected to the primary memory capacity and to the conversionelement via a coupling element which may be controlled externally. Thecoupling element may be in particular a transistor. The connectionbetween the two memory capacities which may be switched externally makesit possible to couple or decouple them alternately, so that chargescontained in the primary memory capacity may be taken over by thesecondary memory capacity, while the latter may then be decoupled fromthe primary memory capacity and the conversion element. A suitablydesigned coupling element may also be used as a charge pump to preventsignificant residual charges from remaining in the primary memorycapacity. Further variants of a “frame transfer” from the primary to thesecondary memory capacity are outlined in the description of the Figuresand in the references cited there. Through the secondary memory capacityand the coupling element it is possible to preset as desired theeffective integration period during which integration of the availablecharge carriers in the conversion element takes place.

The invention also relates to an X-ray apparatus for the production ofe.g. medical X-ray photographs, containing an X-ray source for thecontrolled generation of X-radiation and an (X-ray) detector forspatially-resolved detection of the X-radiation. Here the detector isdesigned in the manner explained above, i.e. it contains one or moresensors with a conversion element and an integration unit, a readoutcircuit and a control means to determine the integration period relativeto the exposure interval. Preferably the detector is developed toconform to the design variants described above. The control means of thedetector are coupled to the X-ray source on the one hand and to thesensors of the detector on the other hand, and are set up to control theactivity periods of the X-ray source during which it emits X-radiation,and the integration periods of the sensors of the detector in such a waythat these periods substantially overlap. In this context a significantoverlap means an overlap of at least 80%, and preferably completecoincidence. With the specified X-ray apparatus it is possible toproduce a series of consecutive X-ray photographs of improved imagequality, since disturbance effects due to residual signals areminimized.

The invention also relates to a method for the detection ofelectromagnetic radiation, in particular X-radiation, comprising thefollowing steps:

-   a) The conversion of the radiation to be detected during an exposure    interval into an electrical signal. The electrical signal may    involve in particular charges released by the radiation or a current    flow carried by it.-   b) The integration of said electrical signal during an integration    period, wherein the integration period substantially overlaps the    exposure interval. The overlap is preferably at least 80%, with    particular preference being given to complete coincidence of the    integration period and the exposure interval.-   c) The readout of the electrical signal integrated in accordance    with step b) during a readout interval. Preferably the readout    interval follows the exposure interval immediately.

The method described may be implemented in particular with the aid ofthe detector described above, in which case the improved image qualitydescribed above is achievable.

The invention is explained below by way of example and with the aid ofthe Figures. Here, identical reference characters in the Figures standfor the same components or factors.

In the drawings:

FIG. 1 shows in schematic form an X-ray apparatus according to theinvention;

FIG. 2 shows the course over time of the current flow I(t) together withthe integrated charge signals Q during the conventional production of asequence of photographs;

F FIG. 3 shows a representation corresponding to FIG. 2 of theproduction of a sequence of photographs according to the invention;

FIG. 4 shows the dependence of a ratio R between the residual signal andthe image signal depending on a decay parameter of the residual signalfor conventional detectors and detectors according to the invention.

The following explanation of the invention relates to detectors for thedetection of X-radiation, but the invention is not limited to thisapplication. Detectors for X-radiation are used in particular inso-called flat dynamic X-ray detectors (FDXD: Flat Dynamic X-RayDetector). Here a distinction may be made between directly convertingdetectors which convert X-ray quanta into an electrical signal in adirect conversion (e.g. from a-Se, PbO, PbI₂ or HgI₂), and indirectlyconverting detectors. Directly converting detector elements will beconsidered below, but without limiting the invention to such elements.

The X-ray apparatus shown schematically in FIG. 1 contains an X-raysource 40 for the generation and emission of X-rays X, together with anX-ray detector 20 for the spatially-resolved detection of theX-radiation X after passage through a body to be examined (not shown).The detector 20 contains a multiplicity of sensors 10 arranged in rowsand columns in matrix form; in the Figure only one of these sensors isshown in detail with its internal circuit diagram. The internalstructure of this sensor 10 corresponds to an embodiment described indetail in WO 00/70864 A1 and the full content of this document isincluded by reference in the present application. An alternativeembodiment of a detector suitable for the implementation of the presentinvention is disclosed in WO 01/57554 A2, and its full content is alsoincluded by reference in the present application. The electronics of thesensor may be based on a-Si:H, poly-Si or crystalline Si.

The sensor 10 has a conversion element 1 which converts X-radiationdirectly (or indirectly) into mobile electrical charge carriers(electron-hole pairs). Connected in parallel to the conversion element 1is a primary memory capacity 2 which, before the start of an exposureinterval T_(exp) is charged up to a defined voltage, and may thensubsequently be discharged through the conversion element 1, inproportion to the quantity of the charge carriers generated.

The primary memory capacity 2 is connected via a transfer transistor 5and a switch transistor 3 to a readout line 8 which runs in column formover the detector surface and leads to readout electronics 11 locatedexternally on the edge of the detector chip. The switch transistor 3 maybe switched over a control line 7 running in row form over the detectorsurface, by addressing electronics 12. When both transistors 3 and 5 areconductive, the readout electronics 11 can determine the amount ofcharge contained in the primary memory capacity 2 and use theinformation contained therein to produce an X-ray image.

Connected between the two transistors 3 and 5 is one side of a secondarymemory capacity 4. By closing or opening of the transfer transistor 5,this may be alternately connected to or disconnected from the primarymemory capacity 2 and the conversion element 1. Consequently theintegration period, over which the secondary memory capacity 4integrates a current I(t) in the conversion element 1, may be presetalternately from outside through the transfer transistor 5. At the sametime the transfer transistor 5 is preferably also used as a charge pumpto prevent any significant residual charges from remaining in theprimary memory capacity. Further details and variants of the “frametransfer” from the primary to the secondary memory capacity are to befound in WO 00/70864 A1 and WO 01/57554 A2. Control of the transfertransistor 5 is effected via lines 6 running in row form over thedetector surface and leading to a control means 30. The latter are alsoconnected to the X-ray source 40.

FIG. 2 shows the course over time of different variables in theconsecutive production of the X-ray images I, II, III and IV, as itoccurs in a method known from the prior art. Only during the secondphotograph II should X-radiation take place, while the preceding I andthe two subsequent photographs III, IV are dark images.

The topmost line of FIG. 2 shows the main instants of time or periodsusing the example of the second photograph II. Photograph II commencesat instant of time t=0 with an electrical resetting of the sensorelectronics. At the same time e.g. the primary memory capacities 2 ofthe detector 20 of FIG. 1 are charged up to a defined voltage. Atinstant of time t_(Xon), the exposure interval commences; its durationis T_(exp) and it ends at the instant of time t_(Xoff)=t_(Xon)+T_(exp).As mentioned (only) during the second photograph II of FIG. 2 doesX-radiation actually take place during the exposure interval. Theexposure interval is followed by the readout phase of duration T_(rd).The photograph is complete at instant of time t_(frm), with this instantof time simultaneously representing the start of the followingphotograph III.

Shown schematically in the second line of FIG. 2 is the course of thecurrent I(t) in the conversion element 1 of FIG. 1. Here it is assumedthat before the observed second photograph II an exposure pause hasprevailed so that, up to the start of the exposure interval at instantof time t_(Xon), only a small, static and constantly present darkcurrent I_(dk) is flowing. During the exposure interval T_(exp) thecurrent I(t) rises rapidly due to a signal current I_(sig) correspondingto the number of available charge carriers. However, on termination ofthe exposure at instant of time t_(Xoff), the increased current flowdoes not end suddenly, but instead fades away relatively slowly due tovarious delaying factors. This leads to a residual signal current I_(ae)(“after exposure”), which extends beyond the observed second photographII to the subsequent photographs III, IV, . . .

In the middle line of FIG. 2, the current curve is shown once again,with the areas covered by it during the individual photograph periodsshown hatched. The contents of the areas correspond to the charge signalaccumulated in the memory capacities of a sensor by integration. Thecourse over time of the charge accumulation Q_(pixel)(t) is shown in theline below. Finally the last line of FIG. 2 shows the ultimatelyread-out signal Q_(rd) which is taken from readout electronics of thememory capacity of the sensor element.

In the conventional procedure shown, the sensor of the detector isactive for most of the time, i.e. from the instant of time t=0 until theend of the photograph t_(frm). During this time interval the currentI(t) flowing in the conversion element of the sensor is integrated andthe resultant charge signal is stored in the associated memory capacity.In the example shown, the integration period T_(int) coincides with theduration of the complete photograph, T_(int)=t_(frm), which is the casee.g. for the last pixel of an array. The integration period T_(int) istherefore (distinctly) longer than the exposure interval T_(exp). Theresultant amount of charge in the pixel memory capacity is accordinglycomprised of various elements: the actual signal current I_(sig),together with the currents flowing before and after the X-ray exposure.Ideally the second contribution corresponds to the dark current I_(dk),and its effect may easily be compensated for by subtraction of a darkimage from the image taken.

In the case of real detectors, though, the effects known as residualsignals occur, as mentioned earlier, and they can not be corrected bysimple subtraction. These effects are based on a slow decay of thesensor current I_(ae) after the end of an X-ray exposure at instant oftime t_(Xoff). Expressed in general terms, the sensor current I(t) at agiven instant of time t depends on the incident radiation and theprevious history of the detector. With reference to the variables shownin FIG. 2, the amount of charge stored in a pixel during (a) a darkimage, (o) an exposure image, and (c) a dark image after exposure may beexpressed as follows: $\begin{matrix}{Q_{dk} = {I_{dk} \cdot T_{int}}} & (a) \\{Q_{sig} = {{I_{sig} \cdot T_{\exp}} + {I_{dk} \cdot T_{int}} + {\int_{t_{Xoff}}^{t_{frm}}{{I_{ae}\left( {t - t_{Xoff}} \right)} \cdot \quad{\mathbb{d}t}}}}} & (b) \\{Q_{ae} = {{I_{dk} \cdot T_{int}} + {\int_{t_{frm}}^{t_{frm} + T_{int}}{{I_{ae}\left( {t - t_{Xoff}} \right)} \cdot \quad{\mathbb{d}t}}}}} & (c)\end{matrix}$

Here, for the sake of simplicity, a constant signal current I_(sig) anddark current I_(dk) have been assumed. After subtraction of the darksignal Q_(dk), the variable R may be defined as the ratio between theaccumulated pixel charges in the first photograph after exposure and theaccumulated pixel charges during exposure.$R = {\frac{Q_{ae} - Q_{dk}}{Q_{sig} - Q_{dk}} = {\frac{\int_{t_{frm}}^{t_{frm} + T_{int}}{{I_{ae}\left( {t - t_{Xoff}} \right)} \cdot \quad{\mathbb{d}t}}}{{I_{sig} \cdot T_{\exp}} + {\int_{t_{Xoff}}^{t_{frm}}{{I_{ae}\left( {t - t_{Xoff}} \right)} \cdot \quad{\mathbb{d}t}}}} = \frac{{\exp\left( {- \frac{t_{frm} - t_{Xoff}}{\tau}} \right)} \cdot \left( {1 - {\exp\left( {- \frac{T_{int}}{\tau}} \right)}} \right)}{\frac{T_{\exp}}{\tau} + 1 - {\exp\left( {- \frac{t_{frm} - t_{Xoff}}{\tau}} \right)}}}}$

Here an exponential decay of the sensor current on completion of theX-radiation has been assumed, i.e.:${I_{ae}(t)} = {I_{sig} \cdot {\exp\left( {- \frac{t}{\tau}} \right)}}$

Since the charge resulting from the build-up of charge after aphotograph is integrated over a long period of time, while the immediateexposure signal occurs only during the shorter exposure time T_(exp),the ratio R assumes a relatively high value.

To reduce the effect of the residual signal coming from the sensor it isproposed according to FIG. 3 to store only the charge generated duringthe effective exposure time window. In other words the integration timeT_(int)* is made equal to the exposure time T_(exp). Such control of theintegration time may be achieved by sensors which implement a “frametransfer” function (see FIG. 1 and e.g. WO 00/70864 A1, WO 01/57554 A2).As the following calculations show, such control of the integration timeleads to minimization of the residual signal effects in the X-ray image.

FIG. 3 shows the variables and courses of time in application of theproposed procedure “T_(int)*=T_(exp)” corresponding to FIG. 2. In thiscase the variables calculated above (for distinction now marked with anasterisk *) read as follows: $\begin{matrix}{Q_{dk}^{*} = {I_{dk} \cdot T_{\exp}}} & (a) \\{Q_{sig}^{*} = {{I_{sig} \cdot T_{\exp}} + {I_{dk} \cdot T_{\exp}}}} & (b) \\{Q_{ae}^{*} = {{I_{dk} \cdot T_{\exp}} + {\int_{t_{frm} + t_{Xon}}^{t_{frm} + t_{Xoff}}{{I_{ae}\left( {t - t_{Xoff}} \right)} \cdot \quad{\mathbb{d}t}}}}} & (c)\end{matrix}$

After subtraction of Q_(dk)*, the ratio between the signal after theexposure image and the X-ray signal in the exposure image may beexpressed by:$R = {\frac{Q_{ae}^{*} - Q_{dk}^{*}}{Q_{sig}^{*} - Q_{dk}^{*}} = {\frac{\int_{t_{frm} + t_{Xon}}^{t_{frm} + t_{Xoff}}{{I_{ae}\left( {t - t_{Xoff}} \right)} \cdot \quad{\mathbb{d}t}}}{I_{sig} \cdot T_{\exp}} = \frac{{\exp\left( {- \frac{t_{frm} - T_{\exp}}{\tau}} \right)} \cdot \left( {1 - {\exp\left( {- \frac{T_{\exp}}{\tau}} \right)}} \right)}{\frac{T_{\exp}}{\tau}}}}$

The above value corresponds to the minimal value which can be obtainedfor a given sensor material.

FIG. 4 shows the reduction in the residual signal obtainable using theprocedure described, in a presentation of the ratio value R definedabove (ratio between the pixel charge in the first photograph after anexposure photograph and the accumulated pixel charge in the exposurephotograph) compared with the decay time constant τ. Here the uppercurve 100 corresponds to a conventional flat dynamic X-ray detector(FDXD), while the lower curve 200 belongs to a detector operatedaccording to the invention. The data presented were obtained on thebasis of a photograph duration of 40 ms and an exposure interval T_(exp)of 10 ms, commencing 5 ms after an electrical reset. According to theresults shown in FIG. 4, the residual signal in a photograph afterexposure may be reduced from 2% to 1.3% e.g. using a sensor materialwith a decay time constant of 8 ms.

1. A detector (20) for the detection of electromagnetic radiation, inparticular of X-radiation (X), in an exposure interval (T_(exp)),comprising: a) One or more sensors (10) with a conversion element (1)for the conversion of electromagnetic radiation (X) into an electricalsignal (I(t)), and with an integration unit (2, 4) for the integrationof the aforementioned electrical signal over an integration period(T_(int)*); b) A readout circuit (11, 12) for the readout and processingof the integrated electrical signal (Q_(dk)*, Q_(sig)*, Q_(ae)*) fromthe sensor (10) during a readout interval (T_(rd)) which follows theexposure interval (T_(exp)); c) A control means (30) coupled to thesensor (10) and set up to determine the specified integration period(T_(int)*) so that it substantially overlaps the exposure interval(T_(exp))
 2. A detector as claimed in claim 1, characterized in that thecontrol means (30) are set up so as to make the integration period(T_(int)*) and the exposure interval (T_(exp)) substantially coincident.3. A detector as claimed in claim 1, characterized in that the readoutelectronics (11) are set up so as to compensate for a dark value(I_(dk)) of the electrical signal (I(t)).
 4. A detector as claimed inclaim 1, characterized in that the conversion element (1) is set up forthe direct or indirect conversion of X-radiation (X) into electricalcharges.
 5. A detector as claimed in claim 1, characterized in that theintegration unit has a primary memory capacity (2) connected in parallelwith the conversion element (1).
 6. A detector as claimed in claim 5,characterized in that the integration unit has a secondary memorycapacity (4) which is coupled to the primary memory capacity (2) and tothe conversion element (1) via an externally controllable couplingelement (5).
 7. An X-ray apparatus comprising an X-ray source (40) and adetector (20) as claimed in claim 1 for the detection of X-radiation,wherein the control means (30) of the detector are coupled to the X-raysource (40) and the sensors (10) of the detector (20), and are set tocontrol the activity times (T_(exp)) of the X-ray source (40) and theintegration periods (T_(int)*) of the sensors (10) of the detector (20)so that they substantially overlap.
 8. A method for the detection ofelectromagnetic radiation, in particular of X-radiation (X), comprisingthe steps: a) conversion of the radiation (X) during an exposureinterval (T_(exp)) into an electrical signal (I(t)); b) integration ofthe electrical signal (I(t)) during an integration period (T_(int)*)which substantially overlaps the exposure interval (T_(exp)); c) readoutof the integrated electrical signal (Q_(dk)*, Q_(sig)*, Q_(ae)*) duringa readout interval (T_(rd)).
 9. A method as claimed in claim 8,characterized in that the electrical signal is a current flow (I(t))carried by released charge carriers.
 10. A method as claimed in claim 8,characterized in that the integration period (T_(int)*) is substantiallycoincident with the exposure interval (T_(exp)).